The present invention concerns a fiber optic soliton signal transmission system and a method of transmission in such a system that reduces the effect of collision jitter.
Transmitting soliton pulses or solitons in the part of an optical fiber that has abnormal dispersion is known per se. Solitons are pulse signals of sech2 form. With this form of pulse, non-linearity in the corresponding part of the fiber compensates dispersion of the optical signal. The non-linear Schrxc3x6dinger equation models the transmission of solitons.
Various effects limit the transmission of such pulses, such as jitter induced by the solitons interacting with noise present in the transmission system, as described for example in an article by J. P. Gordon and H. A. Haus, Optical Letters, vol. 11, no. 10, pages 665-667. This effect, known as the Gordon-Haus effect or Gordon-Haus jitter, imposes a theoretical limit on the quality or bit rate of soliton transmission.
To exceed this limit it is possible to use synchronous modulation of soliton signals by means of semiconductor modulators. Sliding guide filter systems have also been proposed for controlling the jitter of transmitted solitons (see for example EP-A-0 576 208). Using the Kerr effect in synchronous amplitude or phase modulators to regenerate the signal in the line and the use of saturable absorbers have also been proposed. Synchronous modulation of the soliton signals by a clock signal or clock to correct their jitter is described, for example, in an article relating to intensity modulation by H. Kubota and M. Nakasawa, IEEE Journal of Quantum Electronics, vol. 29, no. 7 (1993), p. 2189 et seq. and in an article relating to phase modulation by N. J. Smith and N. J. Doran, Optical Fiber Technology, 1, p. 218 (1995). The Kubota article proposes using an associated filter or regenerator to control amplitude fluctuations generated by the modulation.
Using wavelength division multiplexing (WDM) to increase the bit rate of fiber optic systems for transmitting soliton signals has also been proposed. In this case it is considered advantageous to use Fabry Perot sliding guide filters, which are entirely compatible with wavelength division multiplexed signals. In contrast, using synchronous modulators or saturable absorbers to regenerate wavelength division multiplexed soliton signals is affected by problems concerning the group velocity differences between the signals on different channels.
An article by E. Desurvire, O. Leclerc and O. Audouin, Optics Letters, vol. 21, no. 14, pages 1026-1028, describes a scheme for allocating wavelengths that is compatible with the use of synchronous modulators. The article proposes allocating wavelengths to channels of the multiplex so that, for given intervals ZR between repeaters, the signals on the various channels, or to be more precise the bit times of the various channels of the multiplex, are substantially synchronized when they reach the repeaters. This makes in-line synchronous modulation of all the channels possible, at given intervals, using discrete synchronous modulators. This technique for allocating the wavelengths of the multiplex is also described in French Patent Application 96 00732 of Jan. 23, 1996 in the name of Alcatel Submarine Networks, which proposes choosing a sub-group of channels that are synchronous not only with the intervals ZR but also with intervals that are sub-multiples of ZR. Other aspects of this wavelength allocation technique are described in articles by E. Desurvire, O. Leclerc and O. Audouin, xe2x80x9cSynchronous WDM Soliton Regeneration: Toward 80-160 Gbit/s Transoceanic Systemsxe2x80x9d, Optical Fiber Technology, 3 pages 97-116 (1997) and by E. Desurvire et al., xe2x80x9cTransoceanic Regenerated Soliton Systems: Designs for over 100 Gbit/s Capacitiesxe2x80x9d, Suboptic ""97, pages 438-447. Patent application FR-A-2 743 964, xe2x80x9cFrench titlexe2x80x9d [A method and apparatus for on-line regeneration of a signal transmitted by wavelength division multiplexed solitons using synchronous modulation, and an optical telecommunications system using the method] is also relevant.
An article by L. F. Mollenauer, S. G. Evangelides and P. J. Gordon, xe2x80x9cWavelength Division Multiplexing with Solitons in Ultra long Distance Transmissions using Lumped Amplifiersxe2x80x9d, Journal of Lightwave Technology, vol. 9, no. 3, pages 362-367 (1991) describes the problem of collisions between solitons in wavelength division multiplexed transmission systems and in particular highlights propagation speed variations induced by collisions. Such variations can induce unacceptable jitter of the solitons at the output of the transmission system. The article explains that variations in the chromatic dispersion of the fiber along the transmission path can compensate the effects of collisions. It therefore proposes using segments with different dispersion characteristics to compensate the effects of collisions on the propagation speed of solitons in a transmission system in which the distance between amplifiers is small compared to the length of collisions.
The above solution is difficult to apply on an industrial scale because of fiber management constraints and the small spacing between amplifiers. Moreover, it does not apply to wavelength division multiplexed transmission systems using wavelength allocation schemes of the type referred to above, because variations in the dispersion of the fiber would disrupt the synchronicity of the bit times at the synchronous regenerators.
An article by A. Hasegawa, S. Kumar and Y. Kodoma, xe2x80x9cReduction of Collision-induced time-jitter in Dispersion-managed Soliton Transmission Systemsxe2x80x9d, Optics Letters, vol. 21, no. 1, January 1996, pages 39-41, proposes a scheme for managing dispersion in the fiber which enables the distance between amplifiers to be increased. That solution is based on a stepped dispersion profile of the fiber approximating as closely as possible an ideal exponential profile. That solution cannot be implemented industrially and causes problems with the frequency allocation scheme referred to above.
The aim of the invention is to solve the problem of optimizing filtering in a soliton signal transmission system using synchronous regeneration. In a single-channel system, as in a wavelength division multiplex transmission system, the invention regulates the increase in noise and reduces amplitude fluctuations induced by synchronous modulation. Another aim of the invention is to solve the problem of managing collisions between solitons in a signal transmission system. The invention avoids the need to vary the dispersion profile of the transmission fiber along the transmission system. It applies with particular advantage to wavelength division multiplexed transmission systems in which the wavelengths are chosen to assure synchronicity of the bit times of the channels at given intervals. The invention proposes a simple solution which limits time-jitter due to collisions between solitons. The invention also applies to single-channel soliton signal transmission systems. In these systems it improves transmission performance.
To be more precise, the invention proposes a fiber optic soliton signal transmission system comprising signal amplifier means and signal regenerator means, inline first filter means and second filter means associated with the regenerator means, the second filter means being different from the first filter means.
In one embodiment the first filter means have a greater bandwidth than the second filter means.
The first filter means are preferably on the output side of the amplifier means.
In another embodiment the second filter means are on the input side of the regenerator means.
The regenerator means advantageously comprise an intensity modulator.
In one embodiment the system is a single-channel system and the second filter means have a bandwidth in the range from one-quarter to one-half that of the first filter means, preferably in the order of one-third that of the first filter means.
In another embodiment the system is a wavelength division multiplexed system and the second filter means have a bandwidth more than two-thirds that of the first filter means.
The invention also proposes a method of transmitting soliton signals including a step of amplifying the signals, a step of regenerating the signals, an in-line filtering step and a regeneration filter step effected by second filter means different from the first filter means used for the in-line filtering step.
The first filter means advantageously have a greater bandwidth than the second filter means.
An in-line filtering step can be effected immediately after an amplification step.
A regeneration filtering step is preferably effected immediately before a regeneration step.
The regeneration step advantageously involves intensity modulation.
In one embodiment the transmission system is a single-channel transmission system and the second filter means have a bandwidth in the range from one-quarter to one-half that of the first filter means, preferably in the order of one-third that of the first filter means.
In another embodiment the transmission system is a wavelength division multiplexed transmission system and the second filter means have a bandwidth greater than two-thirds that of the first filter means.